In all audio cassettes now on the market, the magnetic recording tape travels along the front edgewall which is partially cut away to provide access to the tape. In most audio cassette recorders, a spring contacts the center of the rear edgewall of the cassette to bias it forwardly against a retaining means. In the Olympus "Microcassette" recorder illustrated in FIG. 4 of U.S. Pat. No. 4,352,473 (Satoh et al.), the retaining means is a pair of cylindrical locating pins 52. Although not shown in the patent, each of the locating pins of some recorders is of decreasing diameter toward the base plate so that the force exerted by the spring against the center of the rear edgewall of the cassette is translated almost orthogonally along the tapered locating pins to force the front corners of the cassette toward the base plate. The same spring may extend over the broad top wall of the cassette to exert a biasing force directly toward the base plate thus providing a third point at which the cassette is forced against the base plate. The broad bottom walls of some cassettes have offsets at each of these three points at which molds for making the cassettes have been machined with precision. Some cassette decks have precision reference pads at those three points to provide precision tripod positioning. In spite of such precise positioning, audio cassette decks now on the market do not effectively constrain a mounted cassette from small movements in the plane of those three positioning points. Also, if the spring is damaged or becomes fatiqued, the cassette might not be fully seated at all three positioning points, and the user might not be aware of this.
In most recorders for the larger N. V. Philips "Compact Cassette", a spring member likewise acts against the rear edgewall, but forces the front edgewall against stops. The tape path is located by an elongated boss at each side edgewall which fits into a channel in the deck. The accuracy of such tape positioning depends upon the precision of the plastic housing and its elongated bosses, and since such precision cannot be assured in mass-produced plastic moldings, there can be considerable deviation in tape path from cassette to cassette.
The cassette shown in West German Gebrauchsmuster Pat. No. 1,904,775 issued Nov. 19, 1964 has aligned openings in its broad top and bottom walls for receiving a pair of locating pins 7 and 7a, as in the above-discussed U.S. Pat. No. 4,352,473. However, the spring member 10 fits into a central depression in the rear edgewall and together with washers at the base of each locating pin provides tripod support, assuming that gravity holds the cassette against the washers at the bases of the locating pins 7 and 7a. Nothing is said about this in the Gebrauchsmuster patent.